Heat-softenable toner materials are widely used in imaging processes, such as electrostatography, wherein electrically charged toner is deposited imagewise onto a dielectric or photoconductive element bearing an electrostatic latent image. Often in such processes, the toner is transferred to a surface of another substrate, such as a receiver sheet of paper or a transparent film, for example, where it is then fixed in place to yield the final desired toner image. When heat-softenable toners employing thermoplastic polymeric binders are used, the usual method of fixing the toner in place involves applying heat to the toner once it is on the receiver sheet surface to soften it, so that it bonds to the receiver sheet, and then allowing or causing the toner to cool.
One such well-known fusing system passes the toner-bearing receiver sheet through the nip formed by a pair of opposing rolls, at least one of which, usually referred to as a fuser roll, is heated. The fuser roll contacts the toner-bearing surface of the receiver sheet in order to heat and soften the toner. The other roll, usually referred to as a pressure roll, presses the receiver sheet into contact with the fuser roll. In other fusing systems, the configuration is varied with the fuser roll or pressure roll taking the form of a flat plate or belt. As used herein, the term fuser member is used to denote both cylindrical fuser rolls, plates and belts, and the term pressure member is used to denote both cylindrical fuser rolls, plates and belts.
A fuser member typically has a rigid core covered with a resilient material, which is referred to herein as a base cushion layer. The resilient base cushion layer and the amount of pressure exerted by the pressure member serve to establish the area of contact of the fuser member with the toner-bearing surface of the receiver sheet as it passes through the nip of the fuser member and pressure members. The size of this area of contact helps to establish the length of time that any given portion of the toner image will be in contact with and heated by the fuser member. The degree of hardness of the base cushion layer, often referred to as storage modulus, and stability of the base cushion layer are important factors in establishing and maintaining the desired area of contact.
In some previous fusing systems, it has been found advantageous to vary the pressure exerted by the pressure member against the receiver sheet and fuser member. This variation in pressure can be provided, for example in a fusing system having a pressure roll and a fuser roll, by slightly modifying the shape of the pressure roll. The variance of pressure, in the form of a gradient of pressure that changes along the direction through the nip that is parallel to the axes of the rolls, can be established, for example, by continuously varying the overall diameter of the pressure roll along the direction of its axis such that the diameter is smallest at the midpoint of the axis and largest at the ends of the axis giving the pressure roll a sort of bowtie or hourglass shape. This will cause the pair of rolls to exert more pressure on the receiver sheet in the nip in the areas near the ends of the rolls than in the area about the midpoint of the rolls. This gradient of pressure helps to prevent wrinkles and cockle in the receiver sheet as it passes through the nip. Over time, however, the fuser roll begins to permanently deform to conform to the shape of the pressure roll, and the gradient of pressure is reduced or lost, along with its attendant benefits. It has been found that permanent deformation, also known as creep, of the base cushion layer of the fuser member is the greatest contributor to this problem.
Some external heater rollers for nip forming roller fusers are internally heated. These rollers usually have either an anodized surface or a Teflon surface with very low thermal resistance because of the thinness of these coatings. The thin nature of these coatings does not allow a large contact length when a nip is formed with a fuser roller. A longer nip would allow more heating time for the fusing surface. To achieve a longer nip, an elastomer layer, thicker than the anodized or Teflon coatings, can be applied to the heater roller, but that creates a time delay for the heat energy to reach the heater roller surface because of an increase in thermal resistance due to increased thickness of the elastomer surface. A time delay increases thermal response time when altering the fuser roller temperature for any process reason. This increase in thermal response time can preclude the use of gloss control through fuser roller temperature changes, or gloss and fusion tuning for various receiver types. Various receiver types have different thermal properties that affect gloss and fusion quality. The ability to change the fuser roller surface temperature within the time between consecutive receivers allows fusion and glossing to be tuned to receivers within a document run that are of different types without reducing the productivity of the entire electrostatographic system.